Electricity Assisted Deformation in Metallic Glasses

金屬玻璃在電流輔助下的變形現象

Student thesis: Doctoral Thesis

View graph of relations

Author(s)

  • Pak Man YIU

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date25 Aug 2016

Abstract

When electricity was conducted in metallic glasses as a treatment, researchers found that the mechanical behavior and crystallization kinetics were different from those resulted by conventional annealing. Some attribute the phenomenon to the existence of mechanical force, exerted by the electron flow, which electrons in the electric current will transfer momentum to conductor atoms by collision. This particular phenomenon was applied in assisting rolling and forging of engineering alloys i.e. Electroplastic forming, in which plastic defprmation of metal is assisted by applying electricity. Although application and theory of electroplasticity was well established in crystalline alloys, limited study was done on metallic glasses. Therefore the main objective of this thesis is to investigate the feasibility of applying electroplastic forming in metallic glasses, followed by seeking for experimental evidence of electron wind force existing in metallic glasses.
In chapter 1 to 3 of this thesis we will go through some history and background knowledge of metallic glasses, followed by an introduction to the electron wind force and electroplasticity in metals and alloys, and a summary of literature regarding the influence of electric current on metallic glasses. In chapter 4 to 6, the experiments mainly involves the combination of a pulsed direct current power supply and MTS® universal test system with specially designed fixtures.
In Chapter 4 Fe78B13Si9 metallic glass ribbons were subjected to bending stress and pulsed electric current (i.e electropulsing) was simultaneously applied. The temperature profile was recorded throughout the process. Ribbons were found remain in the bent U-shape after electropulsing. It was found that even joule heating temperature in the specimen was well below its glass transition temperature, deformation was still achievable in the ribbon. Different extent of permanent bending was controllable by varying frequency and duration of the electric pulse and bending stress. This method works by relaxing bending pre-stress in the ribbon by electropulsing, however, it has a drawback of unable to attain full deformation without causing embrittlement.
As an improvement the second setup used a copper punch to deform the ribbon while electropulsing simultaneously. Without exceeding the metallic glass’ glass transition temperature, the Fe78B13Si9 metallic glass ribbons were successfully punch-deformed at a reduced stress. No shear band was observable. The deformation showed sign of electroplasticity but not low viscosity rubbery flow, since the time span was too short and temperature was far too low compared to thermoplastic forming of metallic glasses. A mechanism based on potential energy landscape was proposed to explain that an electric current induced force acted as an assistance to lower the energy barrier to activate shear transformation zones, i.e. the mechanism governing most kind of deformation in metallic glasses. For our proposed mechanism to be valid, a key is to prove the current induced force existed in the sample metallic glasses is a mechanical force.
Therefore in chapter 5, Fe78B13Si9 metallic glass ribbon was electropulsed under tensile stress to reproduce the force fluctuation observed in chapter 4. The fluctuation was believed to be caused by a mechanical force, which acted in the form of pulses corresponding to the pulsed power supply. With the help of MTS® recordings and a laser extensometer, we observed a mechanical vibration in-phase with the force fluctuation in the ribbon. This phenomenon was further observed under a high speed camera of 12,000 fps. It was found that those vibrations recorded acted like a standing wave in a string whenever the current pulse strikes the ribbon. This provided strong evidence that the vibrations and fluctuations observed cannot simply attribute to thermal stress, but an actual mechanical force.
In chapter 6, the experimental setup was elaborated into a compressive stress scenario with a few more specimens, namely the glassy and crystallized form of both Zr55Cu30Al10Ni5 and Fe36Co36B19Si5Nb4 rods, an aluminum foil and a steel screw. This chapter aims to prove that the observations in chapter 5 are generally valid over different stress state and composition – In Fe78B13Si9 metallic glass ribbons, periodic fluctuation of stress was observed during electropulsing. The measured frequency and amplitude of the fluctuation was well correlated to the pulse current applied in metallic glass, however aluminum foil did not show the same response. In the electropulsing compression of those glassy and crystalline rod specimens, careful inspection of revealed their difference in waveform, despite they both showed very similar fluctuation phenomena.
Finally in chapter 7, the findings in chapter 4-6 will be summarized. The inspiration given by them will also be discussed. Future works and elaboration will be suggested as well.